Ron Palillo, who portrayed the goofy high school underachiever Arnold Horshack in the hit 1970s sitcom “Welcome Back, Kotter” with such definitive oddballness that he had trouble for years afterward finding work as an actor, died on Tuesday in West Palm Beach, Fla. He was 63.

Ron Palillo, who portrayed the goofy high school underachiever Arnold Horshack in the hit 1970s sitcom “Welcome Back, Kotter” with such definitive oddballness that he had trouble for years afterward finding work as an actor, died on Tuesday in West Palm Beach, Fla. He was 63.